The concept of the distributed power amplifier dates back to the 1940s, when it was used in the design of broadband vacuum tube amplifiers. With recent advances in microwave integrated circuit and device processing technology, the distributed power amplifier has found new applications in broadband amplifiers. Bandwidths in excess of several octaves are possible, with good input and output matching properties as well as giving a flat gain over its bandwidth. There is however a problem to achieve a sufficient output power and Power Added Efficiency (PAE) over the bandwidth with the solutions available today. A prior art distributed power amplifier comprising 3 transistor cells is shown in FIG. 1.
FIG. 1 shows the principle of a prior art distributed power amplifier solution. The distributed power amplifier operates over a bandwidth B and has an input side 101 with an input terminal IN arranged to receive an input signal and an opposite output side 102 with an output terminal OUT arranged to deliver an output signal.
The input terminal IN located at the input side of the distributed power amplifier is connected to an input end 103 of a gate line 104. Suitable DC-biasing can be applied at a first DC-biasing point 105 located at the gate line and at a second DC-biasing point 106 located at a drain line 112. The gate line 104 comprises m transmission line sections Lgn connected in series, where n is an integer ranging from 1 to m. The n integer is increasing in the direction towards the output side. In the example of FIG. 1, m=4. The gate line starts at the input end 103 with a first DC-blocking capacitor 109 and ends with a second DC-blocking capacitor 110 at an end opposite the input end 103, the opposite gate line end 108. At the opposite gate line end 108 a gate line end load 107 is connected to ground.
The output terminal OUT located at the output side 102 of the distributed power amplifier is connected to an output end 111 of the drain line 112. The drain line comprises m transmission line sections Ldn connected in series, where n is an integer ranging from 1 to m and increasing in the direction towards the output end. The drain line ends at the output end 111 with a third DC-blocking capacitor 113 and starts with a fourth DC-blocking capacitor 114 at an end opposite the output end 111, the opposite drain line end 115. At the opposite drain line end 115, a first drain line end load 116 is connected to ground and at the output end 111, a second drain line load 117 is connected to ground.
Transistors Tr1 to Trm−1, each with a source-118, a gate-119 and a drain terminal 120, are connected to the gate line and the drain line, with the gate terminal being connected the gate line and the drain terminal to the drain line, each source terminal being connected to ground. There is one transistor connection from a point between each pair of transmission line sections Lgp/Lgp+1 on the gate line to a point between each pair of transmission line sections Ldp/Ldp+1 on the drain line where p is an integer ranging from 1 to m−1. In the example of FIG. 1, m=4 which means that p assumes a maximum value of 3.
The transmission line sections Lgn of the gate line are thus successively coupled to gate terminals of transistors Trn. The transistors can e.g. be of FET (Field Effect Transistor) type. The drain terminals of the transistors Trn are also successively coupled to the drain line 112 comprising series coupled transmission line sections Ldn as shown in FIG. 1. An RF (Radio Frequency) signal is fed at the input terminal IN and propagates through the gate transmission line 104, with portions of said signal being coupled to the transistors Trn. This solution has a relatively broad bandwidth by using the distributed/travelling wave principle, well known to the skilled person, where the input RF-signal is successively propagated to a second transmission line through the transistors.
A multioctave power amplifier with the solution as described in FIG. 1 typically has an average Power Added Efficiency (PAE) of 20% over the bandwidth.
In new radar systems and multifunctional systems comprising e.g. radar, communication and electronic warfare a broad bandwidth is used and thus a broad band PAE will be of increasing importance.
There is thus a need to achieve an improved PAE and output power for a distributed power amplifier over the entire bandwidth used by e.g. a radar- or multifunctional system. The bandwidth can be multioctave.